The Short Message Service (SMS) provides a means of sending messages of limited size to and from mobile terminals in 3rd Generation Partnership Project (3GPP) communications systems such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS) and Evolved Packet System (EPS). The provision of SMS makes use of a Service Centre, which acts as a store and forward centre for short messages.
Mobile terminal originated messages are transported from a terminal to the Service Centre, while mobile terminated messages shall be transported from the Service Centre to a terminal. Short messages are typically input to the Service Centre by other mobile users, and are destined for other mobile users, or for subscribers on a fixed network. This is described in detail in Technical Specification 3GPP TS 23.040.
Now, for extending battery lifetime of a mobile terminal, a method known as Discontinuous Reception (DRX) is used where the mobile terminal, oftentimes referred as a User Equipment (UE), only occasionally monitors a Physical Downlink Control Channel (PDCCH) to check if there is downlink data available.
Recently introduced UE types, such as Machine Type Communications (MTC) devices, require extremely efficient power saving, which has led to the introduction of a mode referred to as extended idle mode DRX (eDRX). That is, the UE and the network may negotiate over Non-Access Stratum (NAS) signaling the use of extended idle mode DRX for reducing its power consumption, while being available for mobile terminating data and/or network originated procedures within a certain delay dependent on the DRX cycle value. An eDRX enabled UE can only be reachable by the network (i.e. be paged by the network) during its paging transmission window (PTW), and during that window, the UE uses normal DRX.
In “MT SMS procedures for UEs in extended idle mode DRX”, C4-152115, 3GPP TSG CT4 Meeting #71, Anaheim, USA; 16-20 Nov. 2015, it is described a mobile terminated (MT) short message (SM) delivery from a Short Message Service Centre (SMSC) to a Mobility Management Entity (MME).
Reference is made to the timing diagram of enclosed FIG. 1.                1. The SMSC submits a Send-Routing-Info-for-SM-Request (SRR) command to a Home Subscriber Server (HSS) to retrieve the address of the nodes serving the UE.        2. The HSS replies accordingly by submitting a Send-Routing-Info-for-SM-Answer (SRA) message, indicating the address of the serving MME.        3. The SMSC sends an MT-Forward-Short-Message-Request (TFR) to the serving MME. The SMSC indicates in the TFR a Max Retransmission Time (e.g. 5 to 10 minutes), which indicates for how long the SMSC is ready to store the SM if it cannot be delivered at once due to the UE being in extended idle mode DRX.        4. If the UE is reachable within the Max Retransmission Time provided in the TFR, the MME immediately sends an MT-Forward-Short-Message-Request-Answer (TFA) in step 4a indicating Requested Retransmission Time, i.e. the time at which the SM should be resent by the SMSC. The MME also pages the UE in 4b.        At this stage, the SMSC will not send a Report-SM-Delivery-Status-Request (RDR) to the HSS with the indication that the SM delivery was unsuccessful. Consequently, the HSS will not reply with a Report-SM-Delivery-Status-Answer (RDA) message, and will not store the Service Centre address in its Message Waiting Data (MWD).        5. The UE responds to the paging at which point in time a signalling connection is established between the UE and the MME.        At this stage, the MME does not send a Notify-Request (NOR) to the HSS to signal that the UE has become reachable.        6. The SMSC retransmits the SM at the Requested Retransmission Time which was indicated in the TFA (no Max Retransmission Time is included in this second transmission of the TFR).        
The solution in “MT SMS procedures for UEs in extended idle mode DRX” is described to present the following merits:
a. MT SM for UEs in eDRX does not cause any extra HSS signalling.
b. No issue with short eDRX period (e.g. up to 20 s) of synchronization of messages received at HSS.
c. Single-attempt-delivery SM can be transmitted to the UE if this procedure is used (without the need to use MWD for single-attempt-delivery SM as previously has been specified).
The solution in “MT SMS procedures for UEs in extended idle mode DRX” is further described to present the following drawbacks:
a. SM retransmission is triggered without knowing for certain whether the UE indeed is reachable (e.g. the UE may have detached, the UE may be in a tunnel, the eDRX period may have been renegotiated, etc.).
b. Greater impact on SMSC (extended buffering of MT-SM e.g. for up to 5-10 min).
c. The Mobile Station Not Reachable Flag (MNRF) flag is not set in the HSS, so subsequent MT SM to the same UE from a different SMSC will trigger new MT Forward SM Request to MME for UEs which are unreachable.
d. Introduces a further method to deliver MT SMS for UEs using extended idle mode DRX.
The applicant has identified further drawbacks with this solution as will be discussed with reference to FIG. 2.
As in FIG. 1, the SMSC submits a TFR to the currently serving MME (MME1) to indicate that an SM is to be submitted to the UE. The TFR includes a Max Retransmission Time indicating how long the SMSC is ready to store the SM if it cannot be delivered at once due to the UE being in eDRX.
MME1 correspondingly replies with a TFA indicating Requested Retransmission Time at which the SM should be retransmitted by the SMSC, and will subsequently page the UE at a next paging occasion occurring just shortly before beginning of a next Paging Transmission Window, i.e. just before the indicated Requested Retransmission Time, such that the SM can be timely delivered from the SMSC via MME1 to the UE when the UE wakes of from its eDRX cycle at the Requested Retransmission Time.
Now, a first problem arises if the UE wakes up from its eDRX cycle, thus exiting eDRX mode, for instance due to a UE service request for sending data, and correspondingly initiates NAS signalling towards its currently serving mobility management node, e.g. an MME or a Serving GPRS (General Packet Radio Service) Support Node (SGSN), before having been paged by the serving mobility management node, in this particular example MME1.
NAS signalling is used to convey non-radio signalling between the UE and the MME for a Long Term Evolution/Evolved Universal Terrestrial Access Network (LTE/E-UTRAN) access. For instance, a NAS request may be a Routing Area Update (RAU) request which updates the location of the UE within the network. A UE initiates a RAU when it detects that it enters a new Routing Area, and thus it will contact the core network, either the current serving SGSN or a new SGSN.
Hence, the first problem can be exemplified as occurring in case of an intra SGSN RAU request, i.e. when the UE enters a new Routing Area, but is to remain at the serving SGSN, exemplified in FIG. 2 by SGSN1. When the SMSC attempts to submit the SM to the UE at the TFA specified Requested Retransmission Time, there is a risk that the UE again has entered eDRX mode, and the scheduled MT-SMS delivery will consequently fail. When UE is camping in UTRAN, the eDRX cycle will restart at each Routing Area Update procedure.
When the UE is camping in E-UTRAN and GSM, the paging occasion scheme can be maintained, if the serving MME or SGSN would maintain the eDRX cycle regardless of whether the UE requests to change the eDRX cycle at the TAU/RAU. In this scenario, the issue is less troublesome, but the MT-SMS is nevertheless unnecessarily delayed until the next scheduled paging time, extra signalling is required towards the UE, and UE power consumption increases, which indeed should be avoided.
Further, a second problem arises if the UE initiates a NAS request towards another mobility management node, thereby effecting an idle mode mobility procedure between two management nodes referred to as an inter MME TAU request, the other node being exemplified by MME2 in FIG. 2, before the UE has responded to the paging message from the network. That is, the paging message has not been sent by the first mobility management node since the paging window has not yet occurred.
This would e.g. occur if the UE initiates the MME TAU request after MME1 has reported the Requested Retransmission Time to the SMSC indicating a next paging occasion based on the eDRX cycle, but before the actual paging occasion occurs.
In this case, the SMSC would deliver the MT-SMS to the wrong MME, i.e. MME1, at the TFA specified Requested Retransmission Time.